Degeneration of the vertebral joints may lead to arthrosis (osteoarthritis) of the facets that may manifest itself as a reduction in cartilage thickness, which may lead to the complete disappearance of the cartilage and thus generate friction on degenerated joint facets. Osteophytes which are protrusions of bone and cartilage are very common and develop in areas of a degenerating joint as a reparative response by the remaining cartilage. Such protrusions are largely responsible for limitations in joint motion and can also cause pain. These phenomena have been identified as being responsible for lumbagos and radiculalgia that affect a significant portion of the population.
Hence, there is a need to reduce and/or remove friction on damaged, injured, diseased, or otherwise degenerated joint facets and/or to minimize transfer of the forces normally absorbed by the intervertebral spaces onto the joint facets of neighboring and subjacent vertebra, while retaining mobility of the vertebral joints.
PCT Application WO 06/073593 relates to a device for replacing damaged, injured, diseased, or otherwise unhealthy posterior elements, such as the facet joints of a patient's spinal column. This device includes two stabilizing implants, each implant having two lengthwise members which connect an upper and lower vertebrae, via a respective upper and lower pedicular screws. The two lengthwise members are coupled to each other via coupling means. Movement between the upper and lower vertebrae is permitted by the coupling means used between the two lengthwise members. However, this device does not permit mobility of each member relative to the respective pedicular anchors thus limiting mobility to some extent.
U.S. Pat. No. 7,029,475 relates to a dynamic spine stabilizer device, which includes a support assembly and a resistance assembly associated with the support assembly. Ball joints link this device with pedicular screws that are anchored to upper and lower vertebrae. The junction of the dynamic spine stabilizer and pedicular screws is free and rotationally unconstrained. However, the goal of such a device is not to replace degenerated anatomical structures while facilitating motion, but rather to stabilize and control abnormal spinal motion. Indeed, in use, when the upper and lower vertebrae are moved in flexion, this device is adapted to create resistance to the movement of the spine. Through such mechanism, as the spine moves in flexion from the initial position, the device increases resistance thus stabilizing the movement.
Known devices present certain disadvantages regarding, for instance but not limited to, obstruction, kinematics, mobility, friction, positioning and/or the quality of the anchors. It is difficult, for instance, to ensure that the anchors be of a uniform quality in terms of positioning and strength due to variations in form from one vertebra to another and/or between one patient and another. These difficulties may not only lead to bad kinematics of movement but also to degradation of the functional unit (two adjacent vertebrae).
Therefore, there is a need for a prosthesis that preserves intervertebral mobility, while removing friction from between the joint facets, and which prosthesis includes anchors that adapt to the variations in the anatomy of vertebrae and variations due to the surgical technique itself.